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122                               New Trends in Eco-efficient and Recycled Concrete


         removed from the bottom of the decanter tanks as sewage sludge (SS). In many
         cases, the addition of chemical reagents (e.g., polyelectrolytes, iron or aluminium
         salts) facilitates and accelerates the settling process.
           There are several types of secondary treatments (biological), although the most
         common involve the digestion of organic matter, either by aerobic or by anaerobic
         processes, both based on the action of bacteria. The process results in water with a
         low organic content (lowering of the COD and BOD values below established lim-
         its) and mud with a high-water content.
           Tertiary treatments try to eliminate some compounds, or groups of specific com-
         pounds, such as those related to nitrogen (nitrates and nitrites) and those related to
         phosphorus (phosphates). There are also processes for the removal of some soluble
         organic compounds, such as pharmaceuticals.
           Both types of SS generated in primary treatments and secondary treatments are
         liquid, or semi-solid liquid, and require water removal processes (dewatering by
         centrifugation, plate pressing or bed drying, etc.) so that the generated SS can be
         transported and handled in a convenient way. The solid content ranges in 0.25%
         12% by weight (Metcalf, 1991). In some cases, SS is anaerobically digested and
         then the chemical composition changes in terms of total solid content and volatile
         compounds. The processes for humidity reduction produce a sludge with humidity
         ranging from 80% to 20%, depending on the technology used.
           All of these SS contain, in addition to water, a large percentage of diverse
         organic matter and pathogens. They also contain inorganic components. In these
         inorganic components, we can find various types of salts (such as sulphates and
         phosphates) and minerals, such as quartz, feldspars and also clays, among other
         substances. Typical composition ranges for untreated and digested sludge are
         reported in Table 5.1 (Fytili and Zabaniotou, 2008).




          Table 5.1 Range of chemical compositions for untreated and digested sludges
          Parameter                        Untreated sludge     Digested sludge
          Total dry solids (TS, %)         2 8                  6 12
          Volatile solids (%)              60 80                30 60
          Grease and fats (ether soluble, %)  6 30              5 20
          Proteins (%)                     20 30                15 20
          Nitrogen (N, %)                  1.5 4.0              1.6 6.0
          Phosphorous (P 2 O 5 , %)        0.8 2.8              1.5 4.0
          Potash (K 2 O, %)                0 1                  0 3
          Cellulose (%)                    8 15                 8.15
          Iron (not as sulphide, %)        2 4                  3 8
          Silica (SiO 2 ,%)                15 20                10 20
          Note: All parameters, except total dry solids (TS), are given as a weight percentage of TS.
          Data taken from Fytili, D., Zabaniotou, A., 2008. Utilization of sewage sludge in EU application of old and new
          methods—a review. Renewable Sustainable Energy Rev. 12, 116 140. Available from: https://doi.org/10.1016/j.
          rser.2006.05.014.
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